FIG. 25 is a perspective view of conventional weight sensor 501 disclosed in Patent Literature 1. FIG. 26 is a view of deformable body 101 of weight sensor 501. FIG. 27 is a circuit diagram of weight sensor 501.
Deformable body 101 includes an outer side surface having a circular-cylindrical shape extending along center axis 501A. The outer side surface of deformable body 101 includes a pair of transversely extending strain resistor elements 102, a pair of longitudinally extending strain resistor elements 103, power supply electrode 104, ground (GND) electrode 105, and output electrodes 106. The outer side surface of deformable body 101 further includes circuit pattern 107 for electrically connecting these components, thereby forming a bridge circuit shown in FIG. 27.
The following is a description of the operation of conventional weight sensor 501.
FIG. 28 is a side sectional view of object 502 to which weight sensor 501 is attached. Weight sensor 501 is held between parts of mounting member 108 along center axis 501A. When a load is applied to pressing member 109, and hence a compressive force is applied to deformable body 101 in the axial direction, the resistances of the pair of longitudinally extending strain resistor elements 103 decrease, and the resistances of the pair of transversely extending strain resistor elements 102 increase. These strain resistor elements 103 and 102 extending longitudinally and transversely form the bridge circuit together with power supply electrode 104, GND electrode 105, output electrodes 106, and circuit pattern 107. Therefore, output electrodes 106 output their output signals according to the compressive force applied to deformable body 101. The output signal is output to an external circuit through lead wires which are electrically connected to power supply electrode 104, GND electrode 105, and output electrodes 106. Thus, weight sensor 501 outputs the applied load as an electrical signal.
In conventional weight sensor 501, a compressive force is applied to deformable body 101 along center axis 501A of the cylindrical surface. As a result, deformable body 101 itself prevents deformation of strain resistor elements 102 and 103. This makes the resistances of strain resistor elements 102 and 103 less likely to be changed, thereby reducing an output signal which is output from weight sensor 501 according to the compressive force.
FIGS. 29 and 30 are an exploded perspective view and a sectional side view of conventional weight sensor 601 disclosed in Patent Literature 2, respectively. FIG. 31 is a bottom view of insulating board 311 of weight sensor 601.
Insulating board 311 has substantially a square shape, and is made of stainless steel containing about 0.1 wt % of nickel. As shown in FIG. 31, power supply electrode 312, a pair of output electrodes 313, GND electrode 314, four compressive-side strain resistor elements 315, four tensile-side strain resistor elements 316, and circuit pattern 317 are provided a lower surface of insulating board 311. Power supply electrode 312, output electrodes 313, GND electrode 314, and circuit pattern 317 are made of silver. Power supply electrode 312, the pair of output electrodes 313, GND electrode 314, four compressive-side strain resistor elements 315, and four tensile-side strain resistor elements 316 are electrically connected via circuit pattern 317 so as to form a bridge circuit together. Fixing holes 318 extending between upper and lower surfaces of insulating board 311 are provided near four corners of insulating board 311. Detection hole 319 extending between the upper and lower surfaces of insulating board 311 is provided in a center of insulating board 311. Weight sensor 601 further includes pressing member 320 made of stainless steel containing about 4 wt % of nickel. Pressing member 320 includes contact part 321 and external thread 322. Contact part 321 presses around the upper end of detection hole 319 of insulating board 311. External thread 322 is formed in an outer side surface of pressing member 320.
Anti-rotation projection 323 is provided on an outer side surface of pressing member 320 at the center in a longitudinal direction of pressing part 320. External thread 324 for mounting is provided in the upper part of an outer side surface of pressing member 320. Weight sensor 601 further includes mounting member 325 which is made of metal. Insertion hole 326 into which pressing member 320 is inserted is provided substantially in a center of mounting member 325.
FIG. 32 is a bottom view of mounting member 325. Stopper 327 is formed provided on a lower surface of mounting member 325 around insertion hole 326 stopper 327 by cutting or forging.
Fixing holes 328 extending between upper and lower surfaces of mounting member 325 is provided near four corners of mounting member 325. Sandwiching contact parts 329 are provided on the lower surface of mounting member 325 around fixing holes 328, and project from the lower surface so as to be flush with each other. Hexagonal locking part 330 is provided on the upper surface of mounting member 325, and has a step around insertion hole 326. Anti-rotation projection 323 of pressing member 320 is locked into locking part 330. External thread 322 of pressing member 320 passes through insertion hole 326 of mounting member 325 and is inserted into detection hole 319 of insulating board 311. External thread 322 extending downward is screwed with fixing member 331, which is a nut, so that pressing member 320 can be fixed to mounting member 325.
FIG. 33 is a top view of mounting member 332 made of metal. Stopper hole 333 is provided substantially in the upper surface of mounting member 332 at its center and extends downward from the upper surface. Stopper hole 333 accommodates the lower part of external thread 322 of pressing member 320. Four stoppers 334 are formed on the upper surface of mounting member 332 around stopper hole 333 by forging. Strain resistor elements 315 and 316 on insulating board 311 face the area on mounting member 332 excluding four stoppers 334.
Fixing holes 335 extending between the upper and lower surfaces of mounting member 332 is provided near the four corners of mounting member 332. Sandwiching contact parts 336 are provided on mounting member 332 around four fixing holes 335, and project from the upper surface so as to be flush with each other. Weight sensor 601 further includes fixing members 337, four screws, passing through four fixing holes 328 of mounting member 325, four fixing holes 318 of insulating board 311, and four fixing holes 335 of mounting member 332. Fixing members 337 are screwed with nuts 338 so as to sandwich insulating board 311 between sandwiching contact parts 329 of mounting member 325 and sandwiching contact parts 336 of mounting member 332. The area around detection hole 319 of insulating board 311 can be displaced with respect to mounting members 325 and 332 in the vertical direction.
Weight sensor 601 further includes circuit board 339 having integrated circuit (IC) 340 mounted on a lower surface thereof. IC 340 is electrically connected to power supply electrode 312, output electrodes 313, and GND electrode 314 on insulating board 311 via conductive member 341, which is made of gold wire and silicon rubber.
Weight sensor 601 further includes case 342 which includes connector part 343 projecting outward. Six connector terminals 344 electrically connected to IC 340 are provided in connector part 343.
An operation of conventional weight sensor 601 will be described below.
When a load is applied to pressing member 320 from above, the load causes distortion on the surface of insulating board 311. The distortion causes four compressive-side strain resistor elements 315 on the lower surface of insulating board 311 to receive a compressive stress, and four tensile-side strain resistor elements 316 to receive a tensile stress. These stresses change the resistances of compressive-side strain resistor elements 315 and tensile-side strain resistor elements 316. Strain resistor elements 315 and 316 together form a bridge circuit. Therefore, the changes in the resistances of strain resistor elements 315 and 316 are output to an external computer from output electrodes 313. The computer calculates the load applied to insulating board 311 based on the changes in the resistances.
When an excessively large load is applied to pressing member 320 from above, stoppers 334 of mounting member 332 contact insulating board 311, thereby preventing plastic deformation of insulating board 311. Similarly, when an excessively large tensile load is applied to pressing member 320, stopper 327 of mounting member 325 contacts insulating board 311, thereby preventing plastic deformation of insulating board 311.
Stoppers 327 and 334 are formed by cutting or forging, and raise manufacturing cost, accordingly causing conventional weight sensor 601 to be expensive.